It is well-known that in many semiconductor devices, such as VD-MOST and SIT, a high sustaining voltage always accompanies a high specific on-resistance. This is due to the fact that, for a high sustaining voltage, thickness of a voltage sustaining layer should be large and doping concentration of the voltage sustaining layer should be low, so as the peak field does not exceed the critical field for breakdown −EC, which is normally expressed by EC=8.2×105×VB−0.2 V/cm for silicon, where VB is the breakdown voltage of the voltage sustaining layer.
In a uniformly doped n-type voltage sustaining layer between p+-region and n+-region, in order to obtain a minimum specific on-resistance at a given breakdown voltage, a doping concentration ND and a thickness W of the voltage sustaining layer are optimized such that a maximum field is at p+-n-junction and its value is equal to EC, a minimum field is at n+-n-junction and equal to EC/3. For silicon device,ND=1.9×1018×VB−1.4 cm−3  (1)W=1.8×10−2×VB−1.2 μm−2  (2)(see, e.g., P. Rossel, Microelectron. Reliab., vol. 24, No. 2, pp 339-336, 1984).
In a VDMOST shown in FIG. 1A, a field profile in the voltage sustaining layer at VB is shown in FIG. 1B, where a slope of the field versus distance is qND/Es, Es is the permittivity of the semiconductor and q is the electron charge. The change of field through the n-region is qND/Es, 2EC/3. The relation between Ron and VB of a n-type voltage sustaining layer is then expressed byRon=W/qμnND=0.83×10−8×VB2.5Ω.cm2  (3)where μn is the mobility of the electron and μn=710×VB0.1 cm/V.sec is used for silicon.
In order to get even lower Ron at a given VB, some research have been done to optimize the doping profile instead of using a uniform doping, see: [1] C. Hu, IEEE Trans. Electron Devices, vol. ED-2, No. 3, p243 (1979); [2] V. A. K. Temple et al., IEEE Trans. Electron Devices, vol. ED-27, No. 2, p243 91980); [3] X. B. Chen, C. Hu, IEEE Trans. Electron Devices, vol. ED-27, No. 6, p985-987 (1982). However, the results show no significant improvement.